The objectives of this research project are to expand the role of surgery in the treatment of supraventricular dysrhythmias. A search is now underway to find an area in the atrium below the SA node where a dependable atrial rhythm could be generated yet will be of small enough volume to restrict the possibility of re-entry or fibrillation. The experiments include separation of the AV node from the atrium; from all of the atrium except for a portion of the crista terminalis; and, from all of the right atrium but leaving the AV node in the continuity with the left atrium. The hearts will then be examined by serial microscopic sections to relate the electrocardiogram to the surgically altered structure of the conduction system. The response to atrial and ventricular pacing, to vagal stimulation, to Atropine, and to isoproterenol will be determined as well as the capacity of these areas to sustain atrial fibrillation. Finally, the effect on cardiac output of the residual atrial activity in the atrium excluded from the AV node-His system will be compared with that obtained by pacing consecutively the atrium and ventricle to simulate sinus rhythm. A reliable method for producing AV block is now under study. This has proven to be an unexpectedly difficult surgical problem. The need for extensive dissection about the right fibrous trigone is now evident. In order to retain sinus rhythm in the treatment of some supraventricular dysrhythmias, the feasibility of saving a portion of the AV node with only one connection to the SA node will be explored to prevent post-surgical dependency on a pacemaker. A precise delineation of the relationship of the AV node and His to the right fibrous trigone and to the atrial septum from a surgical viewpoint will be done; so that definitive surgery for Kent pathways in this area may be safely interrupted without blocking A to V conduction.